Electronic memories are basically differentiated into two different types of memory. Firstly there are volatile memories such as RAM and memories having short access times, which are often used as a buffer memories, such as cache memories; secondly there are non-volatile memories such as ROM and EEPROM. What is common to all these memories is that they are constructed from regular, orderly structures and are therefore easily, analyzed or manipulated.
It is for this reason that, in security-critical applications, the data which are stored electronically in the memories are protected by suitable cryptographic encryption methods, that is to say the data are stored in encrypted form.
Access to encrypted information in the memories is inevitably accompanied by longer access times to the data, since the data have to be encrypted when they are read into the memory and decrypted when they are read from the memory. This is unavoidable and accepted in the case of external storage media such as smart cards. Memories, however, which have the task of buffer-storing data and making said data available again rapidly, such as buffer memories, caches or the like, cannot employ complex and time-consuming encryption methods on account of their task.
The published patent applications WO 01/54083 A1 and WO 01/53931 A3 proposed, for such memory applications, temporarily valid keywords with which all the data words are encrypted by means of a simple method and are stored in encrypted form in this way in said memory. In this case, the key can be changed or renewed with higher or lower frequency, depending on the security requirement.
After a keyword has been updated, the data content which is situated in the memory and encrypted with the preceding keyword cannot be decrypted. In order to avoid this, when the keyword is changed, the memory content has to be re-encrypted and loaded into the memory again. If the security requirements are high and the key is changed often for this reason, this leads to noticeable dips in performance. The result, then, is a conflict between security and performance.